1. Field of the Invention
This invention relates to the production of a highly accurate and very uniformly distributed mixture of compounds suitable for use in a superconductor. More particularly, this invention relates to the production of a highly accurate and very uniformly distributed mixture of compounds formed by precipitating the mixture from a solution by varying its density, preferably without phase change of the solvent.
2. Description of the Related Art
Since the discovery of superconductivity in 1911, the phenomena of a material being able to conduct electricity with almost no resistance when the material is cooled to a temperature approaching absolute zero (0.degree. K.) has remained an interesting scientific curiosity having few applications which would justify the expense of maintaining the necessary liquid helium cooled system.
Recently, however, superconducting ceramic materials have been produced which exhibit this phenomena at much higher temperatures, e.g., in some cases even higher than the boiling point of liquid nitrogen which boils at about 77.degree. K. The ability to produce superconductivity, for example, in a material cooled by liquid nitrogen completely changes the economics which have previously restricted the applications to which superconductivity could be applied.
These new ceramic materials are sometimes referred to as triple-layer perovskite compounds because of the crystallography of the resulting structure; or 1-2-3 compounds because of the atomic ratios of 1 atom of a first element such as, for example, a rare-earth (Lanthanum series) element such as lanthanum or yttrium, 2 atoms of one or more alkaline earth metals such as barium, calcium, or strontium, and 3 atoms of copper. The superconducting ceramic also contains from 6.5+to 7- atoms of oxygen which may be referred to as O.sub.(6.5+.times.) where .times. is greater than 0 and less than 0.5, resulting in a chemical formula such as, for example, YBa.sub.2 Cu.sub.3 O.sub.( 6.5+.times.).
The prevalent method used to produce this type of superconducting ceramic is to mechanically mix powders of the oxides or carbonates for example, of the respective rare earth, alkaline earth metal, and copper elements in the 1-2-3 structure of the superconductor, calcine the mixture to remove water and/or other volatiles, and then fire the powder mixture in an oxygen atmosphere at a temperature sufficiently high to produce the desired superconducting phase.
However, the mixing process, usually in a ball mill, takes many hours, and sometimes introduces impurities from the balls or the ball milling vessel.
The inadequacies of such a mixing process is evidenced by the variations in compositions of the resulting fired ceramic material, and consequently, variations in properties. If the mixture composition is not accurate, and/or the sintering temperature is too high, the superconductor apparently becomes distributed as small islands embedded in a non-superconducting material.
It is, therefore, apparent that to reproducibly make superconducting material from a mixture of materials, such as the mixture of compounds used in forming such 1-2-3 superconductors, the powdered mixture prepared for sintering must be of the highest purity, i.e., be formed from high purity materials and have minimal contamination introduced through the mixing process, and the powdered mixture must have the respective reactants accurately and uniformly distributed throughout the powdered mixture to the greatest degree possible.
The desire to achieve such purity and uniformity of mixture have lead to attempts to replace the ball milling form of mixing by precipitation of the powdered mixture from a solution. However, these attempts have met with difficulties because of the very large differences in solubility of the different ingredients at the processing conditions employed in such precipitation processes, resulting in non-uniform precipitation of the various compounds needed in the mixture to form the superconductor.
It would, therefore, be desirable to provide a more satisfactory method of forming the desired uniform and stoichiometrically accurate mixture of high purity reactants used in forming the superconducting material.